The critical current of a superconductive material is the current above which the material is normal and below which the material is superconducting, at a specified temperature and in the absence of external magnetic fields. It is obviously advantageous for a material to have as high a critical current value as possible in order to minimize the amount of external cooling to which it must be subjected.
The prior art has described several attempts to improve the properties of superconductive materials by changing the orientation of the particles comprising them. Thus, at least as early as 1988, an attempt was made to provide a Ba.sub.2 YCu.sub.3 O.sub.7-x material with improved current density properties. In his thesis entitled "Feasibility of Grain-Oriented Processing of Ba.sub.2 YCu.sub.3 O.sub.7-x Ceramic Superconductor by Molten Salt Synthesis" (submitted to the Faculty of Alfred University in May of 1988), Christopher T. Decker disclosed (at page 1 of the thesis) that "Single crystals of Ba.sub.2 YCu.sub.3 O.sub.7-x are difficult to grow in large sizes. Conventional methods of ceramic fabrication . . . result in random orientation of the particles resulting in averaged isotropic properties of the final ceramics. The existing anisotropy in the current density of Ba.sub.2 YCu.sub.3 O.sub.7-x warrants a fabrication process of polycrystalline ceramics which would yield net single crystal-like properties in at least one direction with the aim to enhance the current density of the ceramic."
Decker's thesis described an experiment in which reagent grade barium carbonate, yttrium oxide, and copper oxide were combined in molar proportions of Ba.sub.2 YCu.sub.3 O.sub.7-x and this powder mixture was combined with an equal weight of an eutectic mixture of sodium chloride--potassium chloride. This mixture was then milled for 6 hours, dried, and fired at a temperature of from 700 to 900 degrees Centigrade for 2 hours. The fired material was then analyzed by x-ray-diffraction. It was found, on ". . . comparing the XRD pattern with that of Ba.sub.2 YCu.sub.3 O.sub.7-x, . . . that Ba.sub.2 YCu.sub.3 O.sub.7-x was not present (see page 5 of the thesis)." At page 11 of his thesis, Decker concluded that "The formation of Ba.sub.2 YCu.sub.3 O.sub.7-x in the presence of molten salts of Na-K, Ba-C and C belonging to chloride and sulfate systems does not appear to be feasible in the temperature range of 400-900 degrees Centigrade. On further stability analysis of the solid-state reacted Ba.sub.2 YCu.sub.3 O.sub.7-x powders in molten salts it was found that the presence of a eutectic liquid accelerates the decomposition of Ba.sub.2 YCu.sub.3 O.sub.7-x due to the accompanying increase in diffusion . . . ."
A similar conclusion was reached in 1989 in an article entitled "Grain Orientation in high T.sub.c Ceramic Superconductors by Sudhakar Gopalakrishanan and Walter A. Schulze, which was published in "Superconductivity and Applications," edited by H. S. Kwok et al. (Plenum Press, New York, 1989), at pages 411-418. The authors disclosed that it ". . . was noted from our experiments with the 213 powders, that 213 phase degraded with the salt synthesis. The formation of Ba.sub.2 YCu.sub.3 O.sub.7-x in the presence of molten salts of Na, K and Li does not appear feasible in the temperature range of 400.degree. C.-900.degree. C. (see page 414)."
In the same "Superconductivity and Applications" book, at pages 621-628, an article appeared by D. B. Knorr and C. H. Raeder entitled "Stability of Yttrium Barium Cuprate in Molten Salts." In Table 3 of the article (at page 624), the authors disclosed that the presence of halides, nitrates, sulfates, and mixtures of halides and sulfates all caused some decomposition of the Ba.sub.2 YCu.sub.3 O.sub.7-x phase.
Attempts have also been made to improve the properties of other superconductive materials by contacting the materials from which they are made with inorganic salt. In 1988, Akinori Katsui published an article entitled "Crystal Growth of Superconducting Bi-Sr-Ca-Cu-O Compounds from KCl Solution," Japanese Journal of Applied Physics, Part 2, 1988, 27[5], 1844-5. In the experiment described in this article, appropriate amounts of reagent grade bismuth oxide, strontium carbonate, calcium carbonate, and copper oxide were weighed and mixed to give the composition BiSrCaCu.sub.2 O.sub.y. This powder mixture was then melted in air at 1,100 degrees Centigrade. Thereafter, from 5-10 grams of this mixture and 130-140 grams of potassium chloride were charged into a platinum crucible, and the mixture thus formed was heated at 850-950 degrees Centigrade for at least several hours.
Katsui reported that Ba-Sr-Ca-Cu-O compound thin plates were found in the product. "However, CuO needle-like crystals were grown from the underside surface of the thin plates." X-ray diffraction analysis of the product revealed a pattern which was ". . . similar to that of the starting material, except for a difference in the relative intensities of the diffraction peaks."
In his article, Katsui disclosed that his process does not affect the critical current of the superconductive material. He reported that "Zero resistance was observed at about 80K. This value is almost the same as those reported and those of the crystals grown from a Bi.sub.2 O.sub.3 --SrCO.sub.3 --CaCO.sub.3 --CuO solution . . . This suggests that the properties of the grown crystals were not significantly changed by using KCl as the flux."
It is an object of this invention to provide a process for the preparation of a superconductive material with an increased critical current density.
It is another object of this invention to provide a process for preparing a superconductive material which will not substantially degrade the desired superconductor.
It is yet another object of this invention to provide a tape casting method for the preparation of shaped objects which uses the superconductive material produced by applicants' process.
It is yet another object of this invention to provide a metal-ceramic composite produced by printing a conductive ink onto a material comprised of the superconductive composition of this invention.
It is yet another object of this invention to provide an insulating coating on a superconducting tape comprised of the superconductive material of this invention.
It is yet another object of this invention to provide a process for the preparation of a superconductive material which may be used to prepare relatively large, superconductive ceramic bodies.